This invention relates to the preparation of polymers of ethylene. In one of its aspects this invention relates to the preparation of polymerization catalysts. In another of its aspects this invention relates to the treatment of a catalyst support substrate containing silica to effect a change in the polymer produced using the substrate as part of a polymerization catalyst.
To control the shear response of polyolefins by the treatment of chromium oxide containing catalyst used in the preparation of the polymers is described in U.S. Pat. No. 3,288,767. In that process it was determined that the "shear response"--the response of shear rate to changes in shear stress--of a molten polyolefin could be affected by a hydrothermal treatment of a catalyst comprising microspheroidal silica or silica-alumina on which there had been distributed about 1-5 weight percent of an oxide of chromium at least part of which is in a hexavalent form. The hydrothermal treatment applied during the activation of the catalyst at temperatures ranging up to 2000.degree. F. was used to achieve an influence on the shear response of polymers produced with the influence increasing with decreasing water vapor content as the water vapor concentration was varied in the chromium oxide catalyst activation air during the maximum temperature heating stage. It has now been determined that hydrothermal treatment of the silica-containing substrate itself affects the production of polymers produced using catalyst of chromium oxide supported on the treated silica-containing substrates to increase the shear response in comparison to polymers made with untreated silica-containing substrates.
It is therefore an object of this invention to provide a catalyst system which produces polymers that exhibit increased shear response in comparison to polymers made with untreated catalysts. It is another object of this invention to provide a process in which a catalyst system is hyrothermally treated and to provide a catalyst produced thereby. It is still another object to provide a polymerization process using the catalyst produced using the hydrothermal treatment.
Other aspects, objects and the various advantages of this invention will become apparent upon reading the specification and the appended claims.